In recent years, the quality of images formed by digital image forming apparatuses such as a printer, copying machine, and the like has remarkably improved, and high-quality prints can be easily acquired. That is, any one can obtain required prints by image processes using a high-performance scanner, printer, copying machine, and computer. Furthermore, recent high-performance copying machines pose a problem of illicit copies of documents generated in this way, and a security technique required to prevent, suppress, or trace them has increasingly gained its importance.
For example, in a method proposed by Japanese Patent No. 2,614,369, upon outputting a document using a copying machine, printer, or the like, identification information unique to the copying machine or printer used to output is appended to the document using a color agent which is imperceptible to the human eye, and the document is then output. When the document output in this way is copied later, the appended identification information unique to the copying machine or printer is read from the copied document and is analyzed, thus specifying the copying machine and printer used to output the document.
As described above, it is possible in the prior art to specify the copying machine and printer used to output the document, and to trace illicit copies. However, it is difficult in the prior art to verify if the document is an original.